Recent radio communication systems have employed modulation schemes such as OFDM (Orthogonal Frequency-division Multiplexing) that enables realization of a frequency that is used with high efficiency. In radio communication systems using such modulation schemes, since a radio signal contains an amplitude modulation component and a phase modulation component, the PAPR (Peak to Average Power Ratio) levels of the radio signals become large. If a radio frequency modulation signal that contains an amplitude modulation component is amplified by ordinary class A amplifier or class B class amplifier, the signal needs to have sufficient backoff so as to ensure linearity. Generally, the backoff has to be at least the same as the PAPR level.
In contrast, the efficiencies of class A amplifiers and class B amplifiers become maximum when they operate in the saturation state. Their efficiencies are reversely proportional to the backoffs. The higher PAPR level a radio frequency modulation signal has, the more difficult it is for a power amplifier to improve power efficiency.
Polar modulation power amplifiers that efficiently amplify radio frequency modulation signals having high PAPR levels are known. Polar modulation power amplifiers are used to amplify radio frequency modulation signals that contain the foregoing amplitude component and phase components as polar coordinate components.
FIG. 1 is a circuit diagram showing the structure of a polar modulation power amplifier described as a related art reference in Non-Patent Literature 1.
The power amplifier shown in FIG. 1 includes radio frequency modulation signal input terminal 1101, amplitude signal input terminal 1102, power supply circuit 1103, radio frequency power amplifier 1104, and radio frequency modulation signal output terminal 1105. Power supply circuit 1103 includes linear amplifier 1106, subtraction unit 1107, current detection resistor 1108, hysteresis comparator 1109, switching amplifier 1110, inductor 1111, and power supply terminal 1112.
Input to radio frequency modulation signal input terminal 1101 is a radio frequency modulation signal that has been modulated with respect to amplitude and phase.
Input to amplitude signal input terminal 1102 is an amplitude signal that is an amplitude component of the radio frequency modulation signal that is input to radio frequency modulation signal input terminal 1101. The amplitude signal that is input to amplitude signal input terminal 1102 is amplified by power supply circuit 1103 and then supplied as a power source to radio frequency power amplifier 1104 through power supply terminal 1112. Radio frequency power amplifier 1104 amplifies the radio frequency modulation signal that is input to radio frequency modulation signal input terminal 1101 and then output from radio frequency modulation signal output terminal 1105.
Power supply circuit 1103 shown in FIG. 1 has switching amplifier 1110 and linear amplifier 1106 so as to amplify an input signal in high efficiency and with low distortion. The amplitude signal that is input to amplitude signal input terminal 1102 is linearly amplified by linear amplifier 1106 and then the resultant signal is output. Linear amplifier 1106 is a linear amplifier having low output impedance. The signal amplified by linear amplifier 1106 is output from power supply terminal 1112 through current detection resistor 1108.
Subtraction unit 1107 is connected to both ends of current detection resistor 1108 and then outputs a signal that represents the difference between the output voltage of linear amplifier 1106 and the voltage of power supply terminal 1112. Since subtraction unit 1107 has a high input impedance, it does not consume a large amount of power output from linear amplifier 1106. Current detection resistor 1108 is a resistor having low impedance. The voltage that occurs between both the ends of current detection resistor 1108 is the lowest voltage that can be accepted compared with the voltage of power supply terminal 1112.
The output signal of subtraction unit 1107 is input to hysteresis comparator 1109. Hysteresis comparator 1109 determines whether the polarity of the input signal is positive or negative and then outputs the determined result (pulse signal) to switching amplifier 1110. Hysteresis comparator 1109 has a function that stores an immediately preceding output state and a hysteresis characteristic (hysteresis width V_hys). Thus, if the signal level of the immediately preceding output signal is Low, when the voltage of the input signal becomes equal to or greater than V_hys/2, the signal level of the output signal of hysteresis comparator 1109 is inverted to High. In contrast, if the signal level of the immediately preceding output signal of hysteresis comparator 1109 is High, when the voltage of the input signal becomes equal to or lower than −V_hys/2, the signal level of the output signal of hysteresis comparator 1109 is inverted to Low.
Switching amplifier 1110 amplifies the signal that is input from hysteresis comparator 1109 and then supplies the resultant signal to power supply terminal 1112 through inductor 1111. At this point, a current that is output from switching amplifier 1110 through inductor 1111 is combined with a current that is output from linear amplifier 1106 through current detection resistor 1108 and then the resultant current is output from power supply terminal 1112.
Power supply circuit 1103 shown in FIG. 1 is an amplifier having the advantages of both linearity that is realized by linear amplifier 1106 and high efficiency that is realized by switching amplifier 1110. This is because the output voltage of power supply circuit 1103 depends on the output voltage of linear amplifier 1106 that has a low output impedance and most of the output current is output from switching amplifier 1110 having high efficiency. The current that is output from power supply terminal 1112 is the sum of the output current of linear amplifier 1106 and the output current of switching amplifier 1110. The voltage of power supply terminal 1112 depends on linear amplifier 1106 having a low output impedance. Linear amplifier 1106 outputs a current to power supply terminal 1112 such that the voltage of power supply terminal 1112 is kept at a target value. The output current of linear amplifier 1106 is detected by current detection resistor 1108 and subtraction unit 1107. Hysteresis comparator 1109 adjusts the current that is output from switching amplifier 1110 such that the output current of linear amplifier 1106 does not become excessive. In such a structure, since most of the current that is output from power supply terminal 1112 is supplied from switching amplifier 1110, linear amplifier 1106 needs to correct only the error component of switching amplifier 1110.
FIG. 2 is a circuit diagram showing the structure of a power supply circuit described as a related art reference in Non-Patent Literature 2.
The power supply circuit shown in FIG. 2 includes signal input terminal 1201, linear amplifier 1202, current detector 1203, amplifiers 1204, 1205, and 1207, adder 1206, PWM (Pulse Width Modulation) modulator 1208, switching amplifier 1209, inductor 1210, and signal output terminal 1211.
Input to signal input terminal 1201 is an amplitude signal that is an amplitude component of a radio frequency modulation signal that is input to a radio frequency power amplifier (not shown). The signal that is input to signal input terminal 1201 is supplied to linear amplifier 1202 and amplifier 1204.
Linear amplifier 1202 linearly amplifies a signal that is input through signal input terminal 1201 and then outputs the resultant signal to signal output terminal 1211. Current detector 1203 detects the output current of linear amplifier 1202 and then outputs a signal that represents the current value.
Amplifier 1204 adjusts the amplitude of the signal that is input through signal input terminal 1201 and then outputs the resultant signal. Amplifier 1205 adjusts the amplitude of the signal detected by current detector 1203 and then outputs the resultant signal. Adder 1206 adds the output signal of amplifier 1204 and the output signal of amplifier 1205 and then outputs the resultant signal. Amplifier 1207 adjusts the amplitude of the signal that is output from adder 1206 and then outputs the resultant signal. PWM modulator 1208 modulates the output signal of amplifier 1207 based on the PWM scheme, converts the signal into a one-bit signal, and then outputs the one-bit signal.
Switching amplifier 1209 amplifies the output signal of PWM modulator 1208 and then outputs the resultant signal to signal output terminal 1211 through inductor 1210. The current of the output signal of switching amplifier 1209 is combined with the current of the output signal of linear amplifier 1202. The power supply circuit shown in FIG. 2 has a structure in which the power supply circuit described in Non-Patent Literature 1 is modified such that the switching amplifier is controlled by a PWM modulator.
It is preferable that the power supply circuit of the foregoing polar modulation power amplifier effectively amplify a wide frequency band of signals ranging from a DC (Direct Current) signal to high frequency signals.
However, it is difficult for power supply circuit 1103 shown in FIG. 1 to widen the frequency bandwidth of signals that can be amplified. In power supply circuit 1103 shown in FIG. 1, the input signal is amplified by linear amplifier 1106. Thus, signals having frequencies that cannot be amplified by linear amplifier 1106 are not output to downstream circuits. In other words, the frequency bandwidth of signals that can be amplified by power supply circuit 1103 substantially depends on the frequency bandwidth of signals that can be amplified by linear amplifier 1106. Thus, it is necessary for linear amplifier 1106, that is able to obtain the desired gain in a wide frequency bandwidth, to widen the frequency bandwidth of signals that can be amplified by power supply circuit 1103. Although the operation frequencies of linear amplifier 1106 can be widened, parts that can be used for linear amplifier 1106 are restricted and thereby the cost of linear amplifier 1106 increases. Thus, it is difficult to widen the operation frequencies of linear amplifier 1106.
Even if the operation frequencies of linear amplifier 1106 are widened, the power efficiency of power supply circuit 1103 including linear amplifier 1106 would deteriorate. For example, a square signal amplified, for example, by switching amplifier 1110 shown in FIG. 1 contains very high frequency components. Thus, switching amplifier 1110 needs to have a sufficient gain for high frequency components contained in the square signal. However, it is difficult to realize switching amplifier 1110 that operates at higher frequencies than linear amplifier 1106 does in a wide frequency band. Even if the frequency bandwidth of signals that can be amplified by linear amplifier 1106 are widened, switching amplifier 1110 would not follow the output signal of linear amplifier 1106 in a radio frequency region.
Thus, in the frequency region where switching amplifier 1110 cannot follow the output signal of linear amplifier 1106, the output power of linear amplifier 1106 that has a lower efficiency than switching amplifier 1110 increases and thereby the overall efficiency of power supply circuit 1103 becomes lower.
Moreover, when switching amplifier 1110 performs a switching operation, the parasitic capacitance of a transistor is charged and discharged. As a result, a power loss will increase. The power loss that occurs in charging and discharging for the parasitic capacitance of a transistor is proportional to the on/off frequency of the transistor, namely the switching frequency. Thus, it is difficult for power supply circuit 1103 shown in FIG. 1 to simultaneously satisfy both widening of the frequency bandwidth of signals that can be amplified and improving power efficiency.
As with power supply circuit 1103 shown in FIG. 1, the power supply circuit shown in FIG. 2 has a similar problem.
Since the power supply circuit shown in FIG. 2 includes both amplifier 1204 and linear amplifier 1202, signals that are input to switching amplifier 1209 are not limited to signals that are amplified by linear amplifier 1202. However, since the output of linear amplifier 1202 is connected to signal output terminal 1211 through current detector 1203, the frequency characteristics of linear amplifier 1202 affect the frequency characteristics of the power supply circuit shown in FIG. 2. Since current detector 1203 is set for a value such that linear amplifier 1202 and signal output terminal 1211 are connected with low power loss, this structure is substantially the same as the structure in which the output of linear amplifier 1202 is directly connected to signal output terminal 1211.
Thus, in the power supply circuit shown in FIG. 2, the frequency bandwidth of signals that can be amplified by linear amplifier 1202 substantially becomes the frequency bandwidth of signals that can be amplified by the power supply circuit. Consequently, the power supply circuit shown in FIG. 2 cannot simultaneously satisfy both widening of the frequency bandwidth of signals that can be amplified and improving power efficiency.